An internal electrode used for indicating a fixed reference potential in pH measurement etc., is configured in such a manner that the internal electrode including, for example, silver/silver chloride and the like is immersed in an internal liquid including a high-concentration KCl solution so that this internal liquid may contact a sample liquid via a liquid junction including a porous member such as ceramic, glass and the like.
Accordingly, if a high-concentration KCl solution is used as an internal liquid of a reference electrode, K+ and Cl− flow into a sample liquid side and KCl flows into the sample liquid via the liquid junction, and therefore the KCl concentration of the internal liquid will be reduced. In order to prevent fluctuations of the reference potential due to this reduction of the KCl concentration, it is necessary to frequently fill up and exchange the internal liquid. In addition, in consideration of volatility as a liquid of the internal liquid, there is also a limitation that it is necessary to have a large volume in the support tube which accommodates the internal liquid.
For example, in order to prevent fluctuations of a potential difference between the liquids, clogging of the liquid junction, and reduction of the KCl solution due to an outflow thereof into the sample liquid, the liquid junction includes a gelled hydrophobic ionic liquid configured to be used as a salt bridge while using the KCl solution as the internal liquid, and regarding such a reference electrode, the present applicant has filed prior patent applications (see Patent Literatures 1 and 2).
However, in these reference electrodes, ions of an ionic liquid slightly flow out into a sample liquid from a gelled hydrophobic ionic liquid contacting the sample liquid. Therefore, the ionic liquid in the gel may become a prescribed quantity or lower, and it may possibly be difficult to increase a life cycle of a product.
In order to address such a problem, although it may be considered that a thickness of the gel of the hydrophobic ionic liquid is made large so as to contain a lot of ionic liquid, it is very difficult in manufacturing technique with a great increase in cost to make the thickness and size of the gel large while maintaining, for example, the hardness in a degree capable of keeping a shape only by the gel of the hydrophobic ionic liquid.